1. Field of the Invention
This invention relates to the purification of propylene oxide. More particularly, this invention relates to a process for removing contaminating quantities of methyl formate from impure propylene oxide. Still more particularly, this invention relates to a sequential method for the removal of methyl formate and methanol from propylene oxide.
The epoxidation reaction mixture that is formed when propylene is reacted with tertiary butyl hydroperoxide in solution with tertiary butyl alcohol in the presence of a soluble molybdenum epoxidation catalyst will normally comprise unreacted propylene, propylene oxide, unreacted tertiary butyl hydroperoxide, tertiary butyl alcohol, a soluble molybdenum catalyst and impurities including methyl formate and methanol. The reaction mixture is normally separated by distillation into a plurality of distillation fractions including a recycle propylene fraction, a propylene oxide fraction, a tertiary butyl alcohol fraction, and a heavy liquid distillation fraction containing tertiary butyl alcohol, unreacted tertiary butyl hydroperoxide, heavy impurities and substantially all of the dissolved molybdenum catalyst.
In accordance with the present invention, the impure propylene oxide distillation fraction is brought into contact with a basic ion exchange resin in a treating zone for a period of time sufficient to substantially completely convert the methyl formate to formic acid and methanol. A partially purified substantially methyl formate-free propylene oxide fraction is recovered from the treating zone.
In accordance with a preferred embodiment of the present invention, the partially purified propylene oxide is then charged to an extractive distillation zone where the impure propylene oxide is further purified by extractive distillation with an extractive distillation agent consisting essentially of acetone and water in order to remove methanol.
2. Prior Art
A method for the removal of methanol from propylene oxide by extractive distillation with an aqueous acetone blend is disclosed in Meyer et al. U.S. Pat. No. 4,971,661 which issued Nov. 20, 1990.
It is known to remove methyl formate from propylene oxide by treating the impure propylene oxide with an aqueous base, as shown, for example, by Mitchell et al. U.S. Pat. No. 2,550,847, Lichtenwalter et al. U.S. Pat. No. 3,477,919 and Sanderson et al. U.S. Pat. No. 4,691,035.
It is also known to remove methyl formate from impure propylene oxide by extractive distillation as shown, for example, by Kageyama et al. U.S. Pat. No. 3,838,020.
Robeson et al. U.S. Pat. No. 2,622,060 discloses the removal of methyl formate by extractive distillation employing an aqueous solution of an alkaline compound as the extractant.
Binning et al. U.S. Pat. No. 3,350,417 discloses a method for the production and recovery of propylene oxide by plural stage distillation and caustic addition wherein a methyl formate is removed.
It is known to react a hydroperoxide feedstock such as tertiary butyl hydroperoxide with propylene in solvent solution in the presence of an epoxidation catalyst in order to provide a reaction product comprising unreacted feed components, propylene oxide, t-butyl alcohol, a solvent (which may also be t-butyl alcohol), and impurities (see, for example, Kollar U.S. Pat. No. 3,350,422, Kollar U.S. Pat. No. 3,351,635 and Sorgenti U.S. Pat. No. 3,666,777.
It is also known to separate the reaction product by distillation in order to obtain a plurality of fractions including, for example, a propylene recycle fraction, a propylene oxide product fraction, a tertiary butyl alcohol fraction, etc.
It is also known that methyl formate and methanol are common contaminants for propylene oxide which is removed only with difficulty.
For example, Mitchell et al. U.S. Pat. No. 2,550,847 is directed to a process for separating purified propylene oxide from a crude propylene oxide product contaminated with acetaldehyde, methyl formate, methanol, etc., by treating the crude mixture with an aqueous base followed by recovery of the purified propylene oxide by any suitable means such as by decantation. Mitchell et al. reported a recovery of a product containing 78 to 82 wt. % of propylene oxide which, they stated, could be increased in purity to about 95 to 99% by fractional distillation.
In a process unrelated to the purification of propylene oxide, Goddin et al. in U.S. Pat. No. 2,751,337 disclose a process for separating acetone from a mixture of acetone with methanol and methyl acetate utilizing water as an extractive distillation agent.
Hamlin et al. in U.S. Pat. No. 3,409,513 disclose the hydro-extractive distillation of mixtures comprising acetone, lower aliphatic alcohols and esters of lower aliphatic alcohols with carboxylic acids. It is pointed out by the patentees that acetone, methyl acetate and methanol form an azeotrope boiling at 55.5.degree.-56.5.degree. C. Hamlin et al. propose to recover partially purified acetone from such a ternary azeotrope by liquid-liquid extraction with water followed by hydro-extractive distillation of the aqueous phase in order to obtain a partially purified acetone fraction.
Washall U.S. Pat. No. 3,578,568 discloses a process for removing oxygen-containing impurities such as acetone, acetaldehyde and methanol from impure propylene oxide using a glycol such as ethylene glycol or propylene glycol as an extractive distillation agent.
Burns et al. U.S. Pat. No. 3,715,284 discloses a process for the purification of impure propylene oxide using acetone or a mixture of acetone with methanol which is introduced into a distillation column either below or together with the impure propylene oxide.
Schmidt U.S. Pat. No. 3,881,996 is directed to a distillation sequence employing at least three and optionally four columns for the purification of crude propylene oxide, one of the columns optionally being an extractive distillation column wherein a hydrocarbon such as octane is used as the extractive distillation agent.
Schmidt U.S. Pat. No. 4,140,588 discloses a process for the purification of propylene oxide contaminated with methanol and acetone using water as an extractive distillation agent, the water being introduced into the distillation column above the point of introduction of the crude propylene oxide feed.